The invention relates to an optical amplifier and a resonator.
In a field of a fiber laser device and an optical fiber communication, an optical fiber amplifier is used to increase an intensity of light in some cases. In such an optical fiber amplifier, an amplifying optical fiber doped with rare earth elements is extended or an intensity of at least one of seed light and excitation light input to the amplifying optical fiber is increased, in order to increase an intensity of output light output from the amplifier.
However, when the amplifying optical fiber is extended or the intensity of at least one of the seed light and the excitation light input to the amplifying optical fiber is increased, there may be a nonlinear optical effect such as stimulated Raman scattering and four-wave mixing in the amplifying optical fiber. In order to suppress the stimulated Raman scattering, the four-wave mixing, and the like, the amplifying optical fiber may be shortened and a concentration of the doped rare earth elements may be increased. However, when the concentration of the rare earth elements is increased, a concentration quenching occurs in which an excited lifetime of the rare earth elements is shortened so as to lower an amplification efficiency.
Therefore, in an optical amplifier described in Patent Document 1 listed below, a filter function element, which increases a transmission loss of light generated by the optical amplifier due to the nonlinear optical effect, is provided in the middle of an amplifying optical fiber so as to suppress an effect of the stimulated Raman scattering and the like.    Patent Document 1: Japanese Patent Application Laid-Open No. 2007-123477